Moms, Place, and Low Birth Weight, Part 2: Does Place Matter?

By Kit Frohardt-Lane
This is the second in a three-part series examining correlates of low birth weight in babies born in 2010, 2011, and 2012 in Detroit, Wayne County outside of Detroit, Oakland County, and Macomb County. Low birth weight (LBW), defined as 2500 grams or less1, is a significant contributor to Detroit’s alarmingly high infant mortality rate.2 We offer this analysis in the belief that a better understanding of factors influencing birth weight can help reduce the mortality rate.

The first blog post looked at the associations between birth weight and the mother’s age, education, marital status, ethnicity, and race; the level of prenatal care she received; and the area of residence for women within the city of Detroit.

This second blog post compares the findings for Detroit to those for the “Metro Region” — defined as Wayne County outside of Detroit as well as Oakland County, and Macomb County — on the same characteristics, in effect asking, “Does place matter?”

A glance at the average rates of low birth weight for single births (we exclude multiple births3) in Detroit (12%) and the Metro Region (6%) tells us that, yes, place does matter, but it does not tell us why or for whom the differences occur.

So here we’ll look at whether knowing place of residence (Detroit or the Metro Region) adds appreciably to what we already know; namely, that the overall LBW rate for single births (singletons) is twice as high in Detroit as in the Metro Region and how the factors listed above (from the first blog post) were related to birth weight in Detroit. For example, in the first blog post we showed that, in general, as educational attainment increased for Detroit women, the rate of low birth weight rate decreased. Does the LBW rate for Metro Region women follow the same pattern but at a lower rate?

Source of data

The data for this study came from birth certificate records of babies born in 2010, 2011, and 2012 with the mother’s residence in Wayne, Oakland, or Macomb Counties. The first blog in this series described the source in detail.

Age

In Table 1 and Figure 1, we compare the association between the mother’s age and the birth weight of her infant in the two regions The rate for teens in Detroit having a single birth in 2010-2012 was 12%, in line with the rates for other age groups in Detroit up to age 35. Metro Region teens, however, had a rate 3 to 5 percentage points higher than the rates for other age groups. Because the Metro Region teens giving birth during these three years constituted just 6% of the total singleton births, their rate had little effect on the average Metro Region rate of 6%. And place made much less difference in the LBW rate for teens than for other age groups.

The gap in LBW rates between the two regions widened as age increased. The rate for Metro Region women declined with increasing age while the rate for Detroit women through ages 30-34 stayed nearly constant. The gap reached 7 percentage points for women ages 30-34 and 9 percentage points for women ages 35 and older. To put these wide gaps into perspective, if Detroit women aged 30 and above had had the LBW rate of Metro Region women of the same age, there would have been 563 fewer LBW births to Detroit women over this three-year period: 421 rather than the 984 that occurred. Clearly, for age, the place of residence mattered.

Educational Attainment

Next we turn to the relationship between educational attainment and birth weight. Table 2 and Figure 2 show that except for women with an eighth grade education or less, the low birth weight rate declined – sometimes very little — with increasing levels of education. For the Detroit women, the most noticeable drop (2 percentage points) from the average rate4 did not occur until women had a bachelor’s degree or higher – and that was just 5% of the women having single births during this period. For Metro Region women, graduating from high school was associated with a drop of 2 percentage points in the low birth weight rate. Receiving an associates or higher degree was associated with a further decline of 2 percentage points.

In contrast to age, then, the effect of educational attainment on birth weight was similar in both Detroit and the Metro Region: The higher the level of education, the lower the LBW rate.

Marital Status

We divided the mother’s marital status into three categories: never married, married, and widowed or divorced. The latter two categories accounted for 2% of the Detroit women (509) and 3% of the Metro Region women (2,726). Because the widowed or divorced group is a relatively small group in both areas and one with two different marital situations, we have excluded it from the analysis.

For both Metro Region and Detroit women, marital status made a difference in the rate of low birth weight, with marriage conferring a 3 to 4 percentage point advantage in both areas (Table 3 and Figure 3). That advantage, however, was a smaller contributor to the average LBW in Detroit than in the Metro Region because a far smaller percentage of the Detroit women were married (19%) than of the Metro Region women (68%).

Ethnicity and Race

The Hispanic population constituted 10% of the Detroit population and 5% of the Metro Region population, and there was essentially no difference in their low birth weight rates (see Table 4 and Figure 4). In this sense, place did not matter for the Hispanic rate. In another sense, though, it did matter. We could have expected that the Hispanic LBW rate would be lower even than 6% in the Metro Region simply because of the overall average difference in the two regions’ rates, but it was not.

The Non-Hispanic rates differed by 6 percentage points, however, reflecting the overall average singleton LBW rates of 12% in Detroit and 6% in the Metro Region. This difference is not surprising since the Non-Hispanic population of the Detroit women giving birth to one or more singleton births from 2010-2012 was 90% Black, with its average rate of 13%, while the comparable Non-Hispanic population of Metro Region women was 74% White with an average rate of 5%5. (See Table 5 and Figure 4).

Figure 4 starkly illustrates that both place (Detroit and Metro Region) and race (Black and White) mattered for babies’ birth weight, although for both Blacks and Whites place had less effect than simply being Black or White. Race-place interaction is something we will explore in the third blog post in this series.

While the relationship between the level of prenatal care and birth weight followed the same pattern in the two regions, receiving inadequate care had a greater impact on birth weight for Detroit women than for Metro Region women (Table 6 and Figure 5). In Detroit, receiving inadequate care as opposed to adequate or intermediate care was associated with an increase of 5 points in the percentage of LBW babies. One-sixth (16%) of the singleton babies born from 2010-2012 to Detroit women receiving inadequate prenatal care were low birth weight babies. The singleton LBW rate for Metro Region women receiving inadequate care was lower but still constituted 10% of their singleton births during this three-year period.

Of particular note in Table 6 are the number and percentage of women who received inadequate prenatal care6: an average per year of 1,665 women (18%) in Detroit from 2010-2012 and 3,099 women (9%) in the Metro Region.

Geographical distribution of low birth weight babies in Detroit and the Metro Region

Figure 6 places Detroit in the context of the three-county Metro Region, illustrating the percentage of low birth weight singleton babies born to mothers in the years 2010-2012 by census tract. While Detroit has the greatest concentration of census tracts with comparatively high LBW rates, there are clusters of higher rates along Grand River Avenue in Macomb County; in the Pontiac area; just north of the Detroit boundary; and in southern Wayne County.

Figure 6

Summary

We began this blog post by asking, “Does place matter?” To phrase the question differently, suppose that we want to predict how likely women 30 and older in the Metro Region would be to have a low birth weight baby. We know that overall the average low birth weight rate for 2010-2012 was 6 percentage points higher for women living in Detroit than in the Metro Region (12% vs. 6% respectively). We also know, from the first blog post, the average LBW rate for women age 30 or above living in Detroit. So we can ask the question like this: From these two pieces of information, would we get a reasonable prediction of the LBW rate for women 30 and older living in the Metro Region?

The answer: Probably not. The answer, though, depends on the characteristic being investigated. Place made virtually no difference in the LBW rate for Hispanic women in the two areas (both had a quite low rate: 6%) and small differences for teenagers, women with an eighth grade education or less, and Black women. Area of residence did influence the LBW rate for women aged 20 and above (the rate increased with age in Detroit and decreased or leveled off in the Metro Region) and for women with different levels of prenatal care (the difference was especially large for Detroit women receiving inadequate prenatal care compared to such women in the Metro Region). For education and marital status, the patterns were more likely to match expectations.

In the third blog, we will take this examination of associations between mothers’ demographic characteristics and their children’s birth weight further. Because a major difference between Detroit and Metro Region women giving birth during this period is the racial composition of the two groups, and we know that race can have major implications for an individual’s opportunities and well-being, it makes sense to examine the combinations of place and race on rates of low birth weight for the various characteristics of the mother. In the third blog, then, we construct four place-race groups (Detroit Blacks; Detroit Whites; Metro Region Blacks; Metro Region Whites) enabling us to ask, “Does place trump race in its effect on birth weight?” As the next post will show, for the characteristics we examined, the answer is clear.

Very low birth weight is defined as less than 1500 grams. In this analysis, very low and low are aggregated as “low” birth weight. ↩

Multiple births are excluded because they have a known high rate of low birth weight. ↩

As a result of missing data on educational attainment, the average rate for the entire area shown in the table (here 11% for Detroit) differs from the average rate for the entire group (12% for Detroit). ↩

90% of the Non-Hispanic population of Detroit women giving birth in 2010-2012 were Black, 7% White, and 3% other race, while 74% of the Non-Hispanic mothers in the Metro Region were White, 18% were Black, 6% Asian or Asian Indian, and 2% other race. ↩

The Kessner Index assigns a value of adequate, intermediate, or inadequate to the level of prenatal care a woman received. The Michigan Department of Community Health Division of Vital Records and Health Statistics explains the Index as, “… a classification of prenatal care based on the month of pregnancy in which prenatal care began, the number of prenatal visits and the length of pregnancy (i.e. for shorter pregnancies, fewer prenatal visits constitute adequate care).” ↩

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